Patent GB-190410457-A (GB)
Title: Improvements in and relating to Alternating Current Electric Motors.
Filed: 1904-05-06
Granted: 1905-05-06
Abstract: 10,457. British Thomson-Houston Co., [General Electric Co.]. May 6. Field magnets; motors, controlling; sparking, preventing.-Sparking is prevented at the brushes of alternating-current commutator motors by providing a special coil or winding for opposing or neutralizing the electromotive force produced in the short circuited coils. This coil may also be used to assist the starting-torque. Fig. 3 shows one arrangement. On the stator is fitted a commutating-coil C, in such a position relative to the brush b as to neutralize the electromotive force produced in a coil short-circuited by the brush. The coil C is supplied with a current of the required phase and magnitude, being, in the arrangement shown, in shunt with the main circuit a. A resistance R is arranged in connection with the circuits, and the relative amount supplied to the coil C is controlled by a governor G mounted on the motor shaft, the greater the speed the greater being the resistance in the circuit of the coil C. In another arrangement, a separate exciter is provided for the coil C ; its field is in series with the exciting-coil of the stator, and its armature is driven, through differential gearing, from the motor shaft, so that its speed decreases as that of the motor increases. Fig. 5 shows the application of the invention to a polyphase motor. The coils C are excited from a transformer T, the magnitude of the current being varied by shifting the points of connection of the coils and the transformer. For a single-phase current, an induction motor may be used instead of the transformer T. Fig. 7 shows another arrangement, applied to a repulsion motor in which the stator is supplied through a voltage regulator V. An electromotive force is impressed on the coils C from a transformer P. The ratio of transformation is varied by shifting the contact p on the secondary, which may be varied by a governor &c., as in Fig. 3. It is reversed when the contact p passes the central fixed contact. This serves to compensate for the two fields which are produced in repulsion motors according to the speed, whether above or below synchronism. The commutating- coils may be utilized for increasing the starting- torque by placing the contact p on one side at starting and then moving it to the opposite side, which may be done by a reversing-switch. Fig. 9 shows an arrangement for using the coils at starting with a resistance, when an impedance I is switched in the circuit instead of the resistance R. So far, arrangements have only been described to prevent the sparking due to the " motor current." Fig. 8 shows an arrangement adapted to prevent sparking due to "commutation reactance," in which the electromotive force is in phase with the current. One coil C is connected to a transformer P', while the other C' is in series with the coil short-circuited by the brush b. It is not necessary to have two poles, as a single coil having the correct electromotive force will serve. Fig. 11 shows a suitable stator stamping provided as described. Enlarged slots 4 are provided for the commutating-coils C, and the windings may be arranged as described in Specification No. 1481, A.D. 1904.